In the past, bags for use by consumers, such as shopping bags, have been manufactured from paper. Recently, such bags have been replaced for many purposes by thermoplastic bags, particularly those manufactured from polyethylene. Such polyethylene bags have strength characteristics, particularly when wet, not normally found in paper bags. Additionally, paper bags ordinarily have carrying handles near the open mouth portion of the bag which are constructed from separate handle elements, distinct from the bag structure. In plastic bags, the handles may be formed as integral extensions of the bag sides or walls. Plastic bags of this configuration are sometimes known as "t-shirt" bags.
Plastic bags may be manufactured to include integral tabs which are attached to the main bag body by a perforated area. Individual bags are conveniently formed into stacks with their tabs joining, as by stapling, to form a bag pack or unit. Individual bags may be separated by tearing them away at the perforation.
A bag for use in a bag pack structure is disclosed in U.S. Pat. No. 4,165,832, which describes a plastic bag having handles shaped to be wider at the top to reduce the tendency for the bag handles to curl into a small cross-sectional area in the user's palm. This bag additionally includes stress relief notches at the point of attachment of the handles to the bag mouth to minimize the likelihood of tearing away the handles as a result of stress concentration.
Plastic bags of the type under consideration have typically been manufactured from a continuous tube of thin thermoplastic material, such as polyethylene, by a process including cutting sections of the tube to form the general shape of the bag, folding to form side "gussets" or pleats, and thermal welding to form the handles and the closed bottom of the bag.
Recently, it has become practical to manufacture tubes (also known as webs) of polyethylene having a substantially larger diameter than was previously practical. Such larger tubes may have, for example, three times the circumference (or more) of the tubes previously used in the manufacture of bags. These larger tubes may be sliced rapidly into lengthwise sections from which smaller tubes may be formed simultaneously. For example, a single tube may be cut into three lengthwise sections to form three individual tubes and subsequent bag manufacturing operations can occur simultaneously instead of successively. Up to three times as many bags may be made in the period of time previously required to manufacture one.
Further, the cost of the extruder used to manufacture the tube, or web, is relatively high compared to other components of the plastic bag manufacturing line. Thus, it is highly cost effective to use a single extruder to produce a web which can be slit-sealed into multiple smaller webs, rather than operating multiple extruders producing the smaller webs directly. Thus, thermoplastic bags manufactured from the larger tubes may be made at a substantially lower cost.
Where three smaller diameter webs are formed from a single large web, the two outer webs have one seal and the middle one has two. The seals are positioned at the innermost or central fold of the gussets formed at the sides of the bag. These sealed folds are subjected to substantial stress when the bag is filled during use and represent ideal "tearing lines" along which the bag tends to separate under stress. In fact, the sealed fold at the center of the gusset is the point of greatest vulnerability to failure of the bag in use. Such seals were not present when bags were made from a seamless tube by the prior, less efficient process. Hence, the improved process whereby bags are made more efficiently and inexpensively by forming plural tubes in parallel from a single large diameter tube has typically produced bags which are severely limited in their ability to stand up to the stress encountered in normal use. To counter the effect of the weak, sealed fold, bags are sometimes made from thicker, tear-resistent materials. Of course, these materials are more expensive then the plastic which can be used with a seamless bag. Thus, the savings obtained using slit-sealed multiple webs are offset by the increased cost in materials.